CONCLUSIONS 153 



the tvpe of evidence that would allow one to form a verdict of 

 definite relationships. 



(6) The sixth assumption, that the invertebrates gave rise to the 

 vertebrates, has not been discussed in this book. There are several 

 good reviews on this subject. Thus Neal and Rand (1939) pro- 

 vide a useful and interesting account of the various views that have 

 been suggested to explain the relationship between the inverte- 

 brates and the vertebrates. The vertebrates have been derived 

 from the annelids, arthropods, nemerteans, hemichordates and the 

 urochordates. More recently Berrill (1955) has given a detailed 

 account of the mode of origin of the vertebrates from the urochord- 

 ates in which the sessile ascidian is considered the basic form. 

 On the other hand, almost as good a case can be made to show that 

 the ascidian tadpole is the basic form and that it gave rise to the 

 sessile ascidian on the one hand and the chordates on the other. 

 Here again it is a matter of belief which way the evidence happens 

 to point. As Berrill states, "in a sense this account is science fiction." 



(7) We are on somewhat stronger ground with the seventh 

 assumption that the fish, amphibia, reptiles, birds and mammals 

 are interrelated. There is the fossil evidence to help us here, 

 though many of the key transitions are not well documented and 

 we have as yet to obtain a satisfactory objective method of dating 

 the fossils. The dating is of the utmost importance, for until we 

 find a reliable method of dating the fossils we shall not be able to 

 tell if the first amphibians arose after the first choanichthian or 

 whether the first reptile arose from the first amphibian. The 

 evidence that we have at present is insufficient to allow us to 

 decide the answer to these problems. 



One thing that does seem reasonably clear is that many of the 

 groups such as the Amphibia (Save Soderberg 1934), Reptilia 

 (Goodrich 1916) and Mammalia appear to be polyphyletic grades 

 of organisation. Even within the mammals there is the suggestion 

 that some of the orders might be polyphyletic. Thus Kleinenberg 

 (1959) has suggested that the Cetacea are diphyletic, the 

 Odontoceti and the Mysticeti being derived from separate 

 terrestrial stocks. (Other groups that appear to be polyphyletic 

 are the Viruses, Bacteria, Protozoa, Arthropoda (Tiegs and 

 Manton 1958), and it is possible that close study will show that 

 the Annelida and Protochordata are grades too.) 



